This invention relates to capacity control of a refrigeration compressor, and in particular, to a capacity control device which decreases the power input requirements of the compressor motor as the load on the refrigeration unit decreases.
Mechanical refrigeration units, such as those employed in air conditioning systems, normally operate under varying load conditions. Typically, the units are designed to deliver conditioned air at a temperature of 72.degree. F. at high ambients, such as 95.degree. F. (hereinafter maximum load.) When the refrigeration unit is operating at less than maximum load conditions, it is desirable to reduce the refrigeration producing capacity thereof.
Numerous schemes have been proposed to reduce the capacity of a refrigeration unit operating at less than maximum load conditions to not only reduce the refrigeration producing capabilities of the unit to prevent undesired overcooling of a space being served by the unit, but also to reduce the input power required to operate the refrigeration unit. In effect, a refrigeration unit operating under conditions that require less than 100% capacity should ideally be designed to operate at reduced input power requirements to effectively conserve energy.
An example of a prior art capacity reduction device is disclosed in U.S. Pat. No. 3,578,883. This patent discloses the use of a valve to unload one or more cylinders of a refrigeration compressor when reduced capacity is desired.
The valve employed in the cited U.S. Pat. No. 3,578,883 patent is disposed between the suction manifold of the refrigeration compressor and one or more of the refrigerant compressor cylinders. When it is desired to unload the cylinders, to reduce the capacity of the compressor, the valve disposed within the manifold is placed in a position to terminate flow of the refrigerant gas from the manifold to the cylinders. While this method of achieving capacity control has proven somewhat effective, it has been found that further reductions in power input requirements at reduced loads may be obtained by modulating the valve as compared to operating the valve so it either is in an "open" position whereby full flow of refrigerant passes from the manifold to the cylinder or in a "closed" position whereby total flow of refrigerant gas is terminated.
Test results have indicated that a reduction of the input power requirements of approximately 10% may be achieved by modulating the valve to vary the flow of refrigerant to at least one of the cylinders of the compressor as compared to opening or closing a valve in the manner disclosed in the cited patent, particularly when it is desirable to reduce the capacity of the unit to 20%-40% of its maximum load rating.